If your crust is not already baked, roll it out and fit it into a 9-inch pie plate, then crimp the edges. Line with parchment paper and fill with pie weights, then bake at 400°F for 15 minutes. Remove weights and parchment, then bake another 10-12 minutes until golden. Let cool completely before filling.
Pour the cold water into a small bowl and sprinkle the gelatin evenly over the surface. Let it sit undisturbed for 5-10 minutes to soften and bloom until wrinkled and gel-like.
In a medium saucepan, whisk together the egg yolks and 1/2 cup of the granulated sugar until pale and slightly thickened. Whisk in the pumpkin puree, milk, salt, cinnamon, ginger, nutmeg, and cloves until fully combined.
Place over medium heat and cook, stirring constantly, for 8-10 minutes until thickened enough to coat the back of a spoon and reaches a gentle simmer. Do not let it boil. Remove from heat and whisk in the softened gelatin until completely dissolved.
Pour the custard into a large mixing bowl and cover the surface directly with plastic wrap to prevent a skin from forming. Refrigerate for 30-45 minutes, checking occasionally, until cooled and mounded slightly when dropped from a spoon.
In a clean, dry mixing bowl, add the egg whites and cream of tartar. Beat on medium speed until foamy, then increase to medium-high and gradually add the remaining 1/4 cup of sugar. Continue beating until stiff, glossy peaks form, about 3-5 minutes.
Add one third of the whipped egg whites to the chilled custard and whisk gently to lighten. Add the remaining whites and fold with a rubber spatula, scraping from the bottom up, until no white streaks remain. Pour into the cooled crust and smooth the top.
Cover the pie loosely with plastic wrap and refrigerate for at least 4 hours, or overnight, until fully set and holds its shape when sliced.
In a chilled mixing bowl, beat the cold heavy cream, powdered sugar, and vanilla on medium-high speed until soft to medium peaks form, about 2-3 minutes. Spread or pipe over the chilled pie right before serving.